UPDATE 5:10 PM, 7/20: Clouds have kept temperatures down today across most of Southwest Virginia, and therefore, instability has been lacking, and there has been much less shower and thunderstorm development than projected. There is a line of showers/storms near the Ohio River about to cross into West Virginia that is rather slowly pushing southeastward and may affect our region later this evening, likely after sunset. There are also a few showers and storms developing ahead of that line. Similar to last night, we’ll keep an eye on this on radar through the evening. END UPDATE
UPDATE 8:45 AM, 7/20: The Storm Prediction Center has shifted the slight risk zone for severe storms to locations east of the Blue Ridge, where there will be greater heating and therefore atmospheric instability today. Further west, showers and storms are numerous north and west of the Roanoke Valley this morning (latest radar linked here), and additional storms are likely to redevelop later today as a cold front pushes into the host, sticky air mass in place. Expect splotchy coverage of rainfall — heavy for some, light for many, none for others — rather than areawide general rain. END UPDATE
We have a dozen windy cold fronts each winter – and then we had the Feb. 10, 2008, tree-toppling, wildfire-spreading, all-day windstorm. Similarly, we typically have maybe a half-dozen squall lines each summer, barreling at us from West Virginia — and then, we had the June 29, 2012, derecho and all its unforgettable mayhem. Each of those big windstorms were common seasonal weather events pumped up to a higher level by atmospheric steroids. Thursday night, we were back to the garden variety summer squall line. It looked pretty fearsome on radar coming through West Virginia, and there were a small cluster of wind damage reports that way (and quite a few power outages). But as it came into our part of Virginia, it pushed out a shelf cloud (that dark boiling mass that spread out overhead as the wind picked up) that was actually far more picturesque than we saw on June 29, some breezes gusting to about 40 mph (half that of the derecho), some lightning bolts and a little rain. The instability, lift and upper-level wind flow were just not nearly as great today as they were on June 29, and so we got something rather ordinary that resulted in lots of extraordinary photographs. I shared many of these on Twitter tonight as they came in. Matt Wray of Franklin County e-mailed me this panoramic photo of the shelf cloud, which was shot about the same time as this Doppler radar image from the National Weather Service detected the gust front (circled) moving ahead of the actual storm cells in the squall line. And here is a shot of it I took from the roof garden of The Roanoke Times building as the line becomes visible above the mountains northwest of Roanoke.
The factors that produced Thursday’s storms (there were hit-and-miss storms earlier in the afternoon, too) are still present for Friday. In fact, Friday appears likely to be the peak day for this period of storminess (which probably lingers into Saturday), as a cold front to the northwest begins to push a little more into the hot, juicy air mass around us. The Storm Prediction Center, as of Thursday evening, had all of Virginia in a slight risk of severe storms, with damaging winds as the primary threat. Daytime heating — most highs in the 80s, with some in the low 90s — will help get a lot of storms going in the favorable atmospheric environment as the day progresses. Upper-level winds may be a little stronger on Friday, so a few storms may organize into clusters that could produce some severe winds — probably not a large-scale derecho, but some gusty storms. Heavy rain will also be a threat in the strongest storms — the HPC rainfall map favors locations north of us, and as I’ve said before, it’s a little too smoothed out and idealized in its rainfall depiction, as heavier rain will be spotty and some spots may get missed again. It’s a common summer pattern, after all.